Presently used optical communication light wavelengths are from 1.3 to 1.6 microns. Optical waveguides, in fiber or planar form, generally have dimensions many times the wavelength. Thus, optical structures under typical present configurations can have dimensions from a few microns to about 100 microns depending on optical mode requirements and other factors. Optical transmission generally is based on transmission through a higher index-of-refraction material in a core that is surrounded by a lower index-of-refraction material called the cladding. Light is confined within the core material in appropriate geometries by total internal reflection at the dielectric interface for light propagating through the core. Long range optical communications generally are carried on optical fibers. However, manipulation of the optical signals involves optical devices that connect with the optical fibers. Planar structures can present optical devices in a more compact format.
An explosion of communication and information technologies comprising Internet based systems has motivated a worldwide effort to implement optical communication networks to take advantage of a large bandwidth available with optical communication. The capacity of optical fiber technology can be expanded further with implementation of Wavelength Division Multiplexing technology. With increasing demands, more channels are needed to fulfill the system functions.
Basic characteristics of optical materials comprise surface quality, uniformity and optical quality. Optical quality refers to small enough absorption and scattering loss to achieve desired levels of transmission. Optical quality also comprises the uniformity of optical properties, such as index-of-refraction, and bi-refringence properties. In addition, optical quality is affected by interface quality, such as the interface between the core layers and cladding layers. For silica (SiO2) and several other materials, preferred forms for optical transmission are a glass, while for some other materials single crystal or polycrystalline forms may have the highest quality optical transmission.
Several approaches have been used and/or suggested for the deposition of the optical materials. These approaches comprise, for example, flame hydrolysis deposition, chemical vapor deposition, physical vapor deposition, sol-gel chemical deposition and ion implantation. Flame hydrolysis deposition involves the use of a hydrogen-oxygen flame to react gaseous precursors to form particles of the optical material as a coating on the surface of the substrate. Subsequent heat treatment of the coating can result in the formation of a uniform optical material, which generally is a glass material. Flame hydrolysis and forms of chemical vapor deposition have been successful in the production of glass for use as fiber optic elements and planar waveguides.